This invention relates to the continuous casting of metal, and more particularly to continuous casting procedures and apparatus providing induction stirring within the molten interior of a solidifying ingot as the ingot is cast.
In continuous casting as herein contemplated, the metal being cast is progressively advanced through and beyond a chilled, open-ended mold while undergoing peripheral solidification to provide a solid outer shell for the emerging cast body, which typically has a core or central pool of molten metal extending for a considerable distance beyond the mold. Within the emerging ingot, then, there is a solid-liquid interface that tapers progressively toward the center of the ingot in a downstream direction (away from the mold) along the path of ingot advance. Additional positive cooling is commonly applied to the ingot body beyond the mold to promote solidification of the ingot interior. As casting proceeds, fresh molten metal is supplied to the mold so that a continuous, elongated ingot or strand is produced.
It has heretofore been recognized that induction stirring of molten metal in the central pool of the solidifying ingot has a beneficial effect on ingot microstructure and thereby contributes to reduction or control of such problems as segregation, centerline porosity or shrinkage, and columnar growth in the ingot. In particular, as disclosed in U.S. Pat. No. 3,693,697 (issued Sept. 26, 1972 to Alexander A. Tzavaras, one of the applicants herein), these results are achieved by producing, throughout a stirring zone or region in the path of ingot advance beyond the mold, a moving magnetic field that causes a flow or current of molten metal to sweep along the solid-liquid interface within the ingot in a first longitudinal direction (i.e. a direction parallel to the path of ingot advance) and to return along the center of the ingot in a reverse longitudinal direction. That is to say, an alternating current field is produced adjacent the ingot in the stirring region, for example by energizing a helical coil (extending through the stirring region in surrounding relation to the ingot) with alternating current supplied to have at least two successively different phases in longitudinally successive portions of the stirring region, so as to move the molten metal within the ingot along the described flow path, in accordance with known principles of induction stirring.
It is found, however, that such induction stirring may result in undesired localized segregation of interstitials (e.g. C and S) and of inclusion-forming elements (e.g. Al and Mn, in steel ingots), especially if the stirring region is relatively short. For instance, in an illustrative steelcasting operation with induction stirring provided by a helical coil, use of a three-foot-long coil may cause localized segregation while use of a twenty-foot-long coil, under otherwise comparable operating conditions, does not cause segregation. It would be desirable to achieve assured control or elimination of this segregation problem without regard to stirring region length, at least over a wide range of lengths, and in particular to enable use of a relatively short stirring region (as is frequently advantageous for reasons including economy and convenience) without causing excessive localized segregation.